This invention relates generally to the fabrication of integrated circuits on semiconductor wafers, and more particularly to the logistics of moving wafers for processing during fabrication.
A fabrication plant is usually divided into separate areas for particular portions of the fabrication process. Each processing area is called a bay, and each bay contains equipment that is required to process wafers for a certain purpose. In addition to the equipment, material stockers are present near the bay. A material stocker is a large automated cabinet where wafers are stored while waiting to be processed. The wafers are stored in cassettes, and each cassette can hold up to twenty-five wafers. A lot is a logical grouping of wafers in a cassette. The lot can migrate from cassette to cassette as it progresses through the chain of production processes.
A material stocker can hold hundreds of cassettes and services two bays. When a cassette of wafers is ready to be stored in a material stocker, the operator places the cassette at one of the input ports of the material stocker.
When an operator wishes to retrieve a lot from a material stocker, he issues a request via the application program of the host computer. The operator may need empty cassettes to configure or process the wafers. Empty cassettes may be stored in a material stocker, so the operator must have the capability to request empty cassettes. In addition to manipulation of the wafers, the operator may also need to manipulate, or set up, the equipment for an operation.
Once a lot has been retrieved, and the equipment has been set up, the operation on the wafers by a particular piece of equipment, or "tool," can begin. At this point, the lot is "moved-in" to the operation. This state is indicated to the host application by the operator for the lot. The lot remains in this state until the operation is completed. Once the operation is completed, the operator must perform tests and verifications on the wafers. When all tests and verifications have been performed, the host computer application program must be notified. Wafers may have moved from one cassette to another as a result of the operation. The host application has to be notified of this. The operator then places the cassette of "moved-out" wafers in the material stocker, to await orders as to the location of the next piece of equipment which will perform operations on the wafers.
Present automated material handling systems (AMHSs) do not always select the most efficient path for transporting cassettes from one location to another. To illustrate, in many manufacturing environments there are multiple paths by which a cassette may be moved between locations. The paths include physical tracks on which vehicles carry the cassettes, and stockers are present at various locations along the tracks. The length of time required to move a cassette from one location to another is dependent upon the path selected. The reasons for the different transportation times include variations in physical lengths of paths as well as stockers interposed on the tracks between locations. For manufacturing floor efficiency, it is desirable for cassettes to be transported between locations without unnecessary delays.
Present systems are programmed to move a cassette from one location to another such that either a predetermined path is selected or where there are multiple paths, the selection method alternates between the available paths for successive cassettes. If the same path is always selected to moves cassettes, movement delays may occur as a result of too many cassettes being transported on the path. If the system alternates between available paths, some cassettes may be transported very efficiently, while others have relatively long transport times as a result of, for example, physical path length. It would be desirable, therefore, for selection of a path on which to transport a cassette to result in consistently efficient transport times.